Kendall, that proves the point. You had two "unconnected" grounds. Assuming the turntable was properly polarized, it means that either the outlet where the turntable was connected to, or the amp was connected to, was at a different potential.

What this creates is something called a ground loop. The reason why some of those people mentioned a third wire, is to eliminate that situation. If your turntable had the ground attached directly to the amp, it would have done the same thing.